For prevention of dental caries, fluorides, such as sodium fluoride (NaF) or sodium monofluorophosphate (SMFP), have been used as an ingredient of toothpaste. Recently, sodium monofluorophosphate is favored because sodium fluoride is liable to combine with calcium carbonate or alumina used as an abrasive to deteriorate the concentration of active fluorine.
In quantification of sodium monofluorophosphate in toothpaste, it is required to conduct a pretreatment process of eliminating, if any, interfering ions, such as free aluminum or calcium ions. However, such a pretreatment process takes too much time, requires the use of an apparatus too complicated and bulky, with a risk of explosion due to unskillful manipulation, and leads to a low precision of the measurement results.
The conventional method for quantifying sodium monofluorophosphate in toothpaste involves, as shown in Table 1, a pretreatment process, such as distillation or diffusion, and a subsequent quantification process using spectrophotometry, an ion-selective electrode, ion chromatography (IC), or the like to measure the quantity of sodium monofluorophosphate.
TABLE 1StepMethodProsConsPre-DistillationEasy to removeHazardous,treatmentinterferingprocessing onesubstancessample each timeDiffusionRequires neitherTakes long time andseparatehas low precision inequipment norpresence ofreagentinterfering ionsMeasurementSpectro-Easy to getNeeds to operatephotometrymeasurementagain afterpretreatment, andmuch affected byinterfering ionsIon-selectiveEasy to getSensitive toelectrodemeasurementmeasurementenvironment (pH, ionstrength, etc.)ICMultipleRequires expensiveanalysesequipment andpossibleskillfulmanipulation, andhas errors due topeak overlap whenusing an acid inpretreatment
The distillation method is designed to process only one sample each time and uses a distillation apparatus bulky and complicated as shown in FIG. 1. An unskillful manipulation may risk an explosion by pressure.
The diffusion method takes long time of more than 10 hours to perform a pretreatment process and leads to erroneous measurement when interfering ions exist during the measurement of the sodium monofluorophosphate concentration. To prevent leaks of the generated hydrogen fluoride gas, the measurement apparatus should be hermetically sealed as shown in FIG. 2, which makes it impossible to heat the apparatus for faster generation of hydrogen fluoride and thus requires long time to perform measurements.
The spectrophotometry method, which is based on fluoride ion-specific electrode and colorimetric methods for determining the concentration of sodium monofluorophosphate, uses an acid added to convert sodium monofluorophosphate to a measurable form, that is, free fluoride ions. But, the acid causes a release of aluminum or calcium ions from calcium carbonate or alumina used as an abrasive for toothpaste, so aluminum or calcium ions adversely combine with fluoride ions.
The ion-selective electrode method requires a pretreatment process that involves the use of a total ion strength adjustment buffer (TISAB) and the adjustment of the pH value in order to eliminate interfering ions and adjust the ion strength.
The ion chromatography method, which is used for direct quantification of sodium monofluorophosphate, has the difficulty of accurate measurement for the concentration of sodium monofluorophosphate bonded to the interfering ions and requires expensive equipment and skillful manipulation.
Such a pretreatment process is necessary to quantification of sodium monofluorophosphate, with difficulty of rapid and precise measurements.
Hence, there is a demand for developing a novel pretreatment process for more rapid and precise quantification of sodium monofluorophosphate.